Task: Functions and prototypes

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Prototypes are another core concept in JavaScript. Functions and prototypes interact in many interesting ways.

There will not be much about prototypes in this workshop, but there is one thing it is important to understand, especially when working with arguments: In JavaScript you can borrow existing functions that live on a prototype.

var User = function(age) {
    this.age = age;
};

User.prototype.getAge = function() {
    return this.age;
};

describe('prototype-functions', function() {

it('are usually bound to the context of an object', function() {
    var user = new User(25);
    var age = user.getAge();

expect(age).to.equal(25);
  });

// However, in JavaScript we can actually invoke `getAge` without
  // creating an instance of `User`.

// We can say that we are "borrowing" the getAge function
  // from the User prototype.
  it('can be borrowed and have their context bound at will', function() {
    var age = User.prototype.getAge.call({ age: 35 });
    expect(age).to.equal(35);
  });

});

// PROBLEM:
// Create a function that "borrow" `Array.prototype.slice`
// and returns all the arguments given, except the first.

// NOTE: This has changed in ES2015 (aka ES6), but it is
// still a useful exercise.

function rest() {
}

describe('rest', function() {

it('should return all arguments except the first one', function() {
    var result = rest(1);
    expect(result).to.deep.equal([]);
  });

it('should return all arguments except the first one', function() {
    var result = rest(1, 2, 3);
    expect(result).to.deep.equal([2, 3]);
  });

});

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Available globals

_: Lo-dash.js
log: log-command that logs to view and console

Testing

describe: new test suite (mocha)
it: new test (mocha)
xit: pending test (mocha)
xdescribe: pending scenario (mocha)
before/after: run code before or after all tests (mocha)
beforeEach/afterEach: run code before or after each tests (mocha)
chai: chai.js assertion library